The present invention is directed to a method and apparatus for equipping substrates with micropacks.
The desire for miniaturization of printed circuit boards has led to the fact that surface mounting of components, which is already standard for a long time in the film circuits, is now also being increasingly utilized with printed circuit boards.
The surface-solderable components, which are also referred to in short as SMDs in accordance with the English language term "Surface Mounted Devices" are thereby no longer plugged into holes in the board but are soldered onto the surface of the board. They require 30 to 60% less space. Since, of course, the number of plate through holes is also reduced, one can now count on a total cost reduction in the area of about 50%.
Due to the utilization of the surface mounting for printed circuit boards, the necessity occurs of developing components having modified dimensions and terminals. As a result of this development, what is referred to as a micropack arose as the smallest, flattest and lightest IC structure wherein it is a matter of an integrated circuit existing in ribbon form.
Micropacks are composed, for example, of hermetically sealed chips having solderable contacts and ribbons fashioned in the manner of film strips having tin-plated copper interconnects. A thin glass layer is deposited surface wide on the surface of the chip over the Al wiring and the contact holes of the terminals of the IC structure are then etched free. These are then inturned closed with solderable, overlapping metal layers in the form of humps. Together with the humps, the glass then yields a hermetically tight closure for the chip which is thus fully protected against environmental influences.
A 40 mm wide and 127 um thick, high temperature-resistant polyimide ribbon coated with a thin epoxy resin adhesive serves as an initial material for the manufacturing of the film strip. A window for the chip and perforation holes in accordance with the dimensions of a super-8 film are punched in this ribbon. The polyimide ribbon is then glued to a copper foil which is coated with a photoresist and is exposed with a mask mating with the IC structure. Then it is partially galvanically plated and etched so that a fine Cu terminals for the soldering of the chip and the external connections of the micropack structure, which are later required, will be present.
As a result of the flexible suspension of the IC structure in the micropack and the likewise flexible external connections, only extremely slight mechanical forces can be exerted on the chip due to the different coefficients of expansion of the various materials. This means that the surface mounting of the micropack can be unproblematicaly executed on single-layer or multi-layer substrates of ceramic, glass, epoxy resin, hard paper or polyimide foils.
Pulse-controlled contact soldering is heretofore particularly proven itself as a connecting technology for micropacks. However, reflow, condensation or laser soldering and gluing with electrically conductive adhesives are possible for use in a connecting method. However, the temperature measured at the chip should not exceed 220.degree. C. or, respectively, 260.degree. C. dependent on the particular embodiment.
Since micropacks are supplied in ribbon form, the external connections are first formed before the equipping. Then the micropacks are cut from the ribbon. The appropriate forming and cutting stations can be integrated in the soldering devices.
What are referred to as outer lead bonders are commercially available for processing micropacks and these operate on the same principle. A metal hoop of a pulse soldering die is heated by means of a short current pulse and the solder on the substrate board and the micropack external connections are caused to melt. Only after the solder layer has solidified is the pulse soldering die lifted off and out of contact with the connections. A risk of what are referred to as cold solder locations is thus avoided. The precise positioning of the micropacks on the substrate is undertaken under the microscope with appropriate manipulators.
Further details regarding micropacks can be derived from a company publication entitled "Mikropack eine kompakte IC-Bauform fuer die Oberflaeschenmontage", Order No. B1-B 3166, PA 05 855, S3d 3/85, published by Siemens AG, Bereich Bauelemente, Product Information, Balanstrasse 73, D-8000 Muenchen 80.
Automatic equipping units are employed for the automatic equipping of printed circuit boards or ceramic substrates with surface-solderable components, such as resistors, capacitors, diodes, transistors and the like. These automatic equipment units operate according to what is referred to as the pick and place principle. The feeding of the components is thereby based on the condition in which they are supplied. Feeder modules are available both for bulk goods and magazines, supply tapes, etc. are also available. Vibrating circular conveyor modules are particularly utilized for bulk goods and vibration longitudinal conveyor modules are employed for bar magazines and belt feeder modules are also utilized. An equipping head moves over these feeder modules and over the printed circuit boards or a ceramic substrate stationarily centered in the equipping region. This equipping head is connected to a freely programmable positioning unit having two degrees of freedom. The equipping head fitted with a suction pipette or head for picking up components and with two pairs of forceps for centering the picked up component contains three further freely programmable axes for the function of lifting and lowering the suction pipette or head, centering the components and turning the components. The equipping head picks up the components from the tracks of the feeder modules at the respective removable locations with the assistance of the suction head whereby the component respectively picked up is centered in two axial directions with the assistance of two pairs of forceps, is rotated into the integrated position and is deposited on the printed circuit board or the ceramic substrate in the intended equipping position. Subsequently, the substrates equipped in this fashion are forwarded to a reflow soldering station, whereby the surface mounting of the component is concluded, in particular, by vapor phase soldering.
U.S. Pat. No. 4,135,630, whose disclosure is incorporated by reference thereto, discloses an automatic equipping unit which works on the pick and place principle. This equipping unit comprises an equipping head equipped with a suction pipette or head and two pairs of forceps for centering the component picked up.
For the surface mounting of micropacks by means of a reflow soldering method, such as, for example, vapor phase soldering, the equipping of the micropack together with the other components should be able to be undertaken with the automatic equipping unit set forth above. The equipping of substrates with micropacks in accordance with the pick and place principle, however, is not possible without further modifications since the micropack structure is far too sensitive for centering the component picked up with the suction pipette with the assistance of the two pairs of forceps. With knowledge of the relative position between the micropack and the equipping head, however, a positionally correct placing of the micropack in the programmable equipping position is not possible.